It is a well known observation that the biosynthesis of natural polyamines, such as putrescine, spermidine and spermine, is elevated in rapidly proliferating cells relative to normal quiescent cells. Conversely, it is also known that the depletion of putrescine and spermidine leads to a reduction in cell proliferation.
Ornithine is the metabolic precursor of putrescine, which in turn, is the metabolic precursor of spermidine, which in turn, is the metabolic precursor of spermine. Metabolically, these biochemical conversions are catalysed by the enzymes ornithine decarboxylase, spermidine synthase and spermine synthase, respectively. Additionally, spermidine and spermine synthase enzymes utilize decarboxylated-S-adenosyl-L-methionine as a co-substrate, the reaction product of the S-adenosyl-L-methionine decarboxylase enzyme. Inhibitors of these enzymes, including inhibitors of S-adenosyl-L-methionine decarboxylase therefore, should serve to prevent the biosynthesis of putrescine and the higher polyamines derived therefrom, viz, spermidine and spermine, and should, theoretically, be effective as antiproliferative agents and/or antitumor agents.
However, in the past, the use of irreversible ornithine decarboxylase inhibitors or inhibitors of S-adenosyl-L-methionine decarboxylase, spermidine synthase and spermine synthase have not proven to be totally effective. Thus, for example, putrescine and spermidine are not essential for the maintenance of cell viability as long as the preexisting spermine pool is maintained above a certain critical level. Moreover, a total in vivo inhibition of the decarboxylase enzymes is difficult due to their rapid turnover.
Applicants have discovered a class of compounds which deplete the natural levels of spermine in the cell. These compounds are highly effective inhibitors of cell growth in rapidly proliferating cells. Accordingly, the compounds of this invention are useful as antiproliferative and anti-tumor agents.